The purpose of this three year phase III study is to investigate the therapeutic effect of ibandronate in the long-term treatment of postmenopausal osteoporosis by evaluating the efficacy and safety of intermittent intravenous treatment using the incidence of new vertebral fractures as the primary endpoint. The specific objectives are: To determine whether Ibandronate prevents fractures and bone loss in women with established postmenopausal osteoporosis. To determine the type of side effects associated with the i.v. administration of Ibandronate To determine the amount of Ibandronate in the blood following treatment and to estimate how long the drug stays in the blood